After 180 million years buried in volcanic rock in the southern tip of Sweden, a recently discovered fern fossil looks almost as good as new.

The matchbox-sized fossil is among the best ever preserved: Thin slices viewed under a microscope reveal rounded cells jam-packed in the stem, like water balloons stuffed in a barrel. And inside the cells, within tiny dots of nuclei, the shadowy squiggles of chromosomes appear.

Such exquisite detail was probably preserved when minerals dissolved in hot sa...